New look for nanomotors

Mar 22, 2005

Physicists in the US have built the first nanoelectromechanical device that exploits the effects of surface tension. The "relaxation oscillator" consists of two droplets of liquid metal on a substrate made of carbon nanotubes and can be controlled with a small applied electric field. Alex Zettl and colleagues at the University of California at Berkeley and the Lawrence Berkeley National Laboratory say the device could find use in various nanomechanical applications, including actuators and motors (B C Regan et al. 2005 Appl. Phys. Lett.86 123119).

Surface tension becomes more important as objects become smaller and it is the dominant force on the micron scale and below. This is why, for instance, insects can walk on water whereas humans cannot. Although electric fields are already used to change the surface tension in droplets of liquid in applications such as inkjet printers, it has not until now been harnessed as a source of force.

The nanoscale relaxation oscillator made by Zettl and colleagues consists of a "large" drop of molten indium measuring 90 nanometres across placed close to a smaller droplet some 30 nanometres across (figure 1). Relaxation oscillators typically cycle between a fast "relaxation" phase and a slow "recovery" phase. The Berkeley group begins with the slow part of the cycle by applying an electric field through the substrate, which transfers metal atoms from the larger drop to the smaller one.

Using a CCD camera inside a transmission electron microscope, Zettl and co-workers observe that the flow of metal continues until the smaller drop becomes big enough to touch the larger drop, which is shrinking. When this occurs, a hydrodynamic channel is created between the two drops and the pressure difference created between them drives fluid in the opposite direction, from the smaller drop to the larger one. This sets off the fast phase in the device, during which the larger drop quickly "consumes" the smaller drop, allowing the process to begin all over again (figure 2).

The team found it could increase the frequency of the oscillator by increasing the applied electric field from 1.3 volts to 1.5 volts. Relaxation occurs in about 200 picoseconds and 5 femtojoules of energy is released per relaxation event. According to the Berkeley team this means the device could operate at frequencies approaching the gigahertz range.